This is a fairly common problem that I have seen with many players. I struggled with it for a while. Here are some suggestions to try. Bending at the waist and the disc pivot angle are probably the most most important as they allow you to throw a hyzer straight out rather than having to sweep it out wide.

Bend at the waist.Possible Fix:
When I first heard "bend at the waist," I found it to be quite hard to visualize the proper way of doing this. I found thinking about this in a slightly different way is easier to pick up. Drop your right shoulder lower than your left. When you go through your motion with the right shoulder lower it will bend you at the waist. This position also makes it easier to get the inside of your forearm facing somewhat upwards and making a hyzer angle with the nose down easier to achieve at the point of release.

Check for proper disc orientation and disc pivot angle.Possible Fix:
Disc pivot is a tricky one to explain but I will do my best. Newer players often think that the disc will leave your hand exactly as it is at the point of release. This is true if you are letting go of the disc, but a correct grip and throw will have the disc ripping out of your hand by pivoting off between your last finger on the rim and your thumb. You can simulate this by holding the disc with your grip and pulling it slowly out of your hand. Pause at the last place you find your hand touching and that is your disc pivot point. Notice how the disc lurches a little upwards and to the left. If you hold the disc without any hyzer/anhyzer angle and the nose down as it enters the pivot, the resulting up/left lurch will shift the disc into an anhyzer angle upon release. To have the disc pivot off at a nose-down hyzer angle, you will want to orient the disc with a greater hyzer angle than you want it to pivot out with. You can experiment with angle by simulating a disc pivot and with a little practice you should be able to find the angle that works best for you to have the disc release at your desired hyzer angle.

Roll your wrist under.Possible Fix:
Wrist roll is your tendency to turn your wrist/forearm during or immediately after release. This usually happens when your arm is moving very fast so it might be a tough one to spot. If you go through your motion in slow-motion, open your hand like you are going to shake hands with someone. On a release with no wrist roll your palm should be facing somewhat towards you. This is ideal for a line drive shot. For an extreme sweep or knife hyzer you will want to roll your wrist under and end up with your palm facing down. This will expose the flight plate and give a very hard left fade.

Adjust the angle of your footwork.Possible Fix:
The line your steps take during your x-step is very important based upon what type of shot you want to throw. Walk through your x-step and see how far you move to either side while you step forwards. Most people will try to stay centered on the teepad as they go through their steps. If you want to make it easier to throw a hyzer try starting back left and finishing front right while keeping your forward movement perpendicular to the target, this should open your upper body up more for a cleaner hyzer throw out to the right.

Arch your back during your throw.Possible Fix:
Leaning back a little bit with a slight back arch will help to pull the outter edge of the disc upwards and make an anhyzer angle easier to achieve. This might feel a bit awkward but this is how I've been taught to throw big turnover and roller shots.

Check for proper disc pivot angle.Possible Fix:
Disc pivot is a tricky one to explain but I will do my best. Newer players often think that the disc will leave your hand exactly as it is at the point of release. This is true if you are letting go of the disc, but a correct grip and throw will have the disc ripping out of your hand by pivoting off between your last finger on the rim and your thumb. You can simulate this by holding the disc with your grip and pulling it slowly out of your hand. Pause at the last place you find your hand touching and that is your disc pivot point. Notice how the disc lurches a little upwards and to the left. If you hold the disc without any hyzer/anhyzer angle and the nose down as it enters the pivot, the resulting up/left lurch will shift the disc into an anhyzer angle upon release. To have the disc pivot off at a more extreme anhyzer angle, you will want to orient the disc accordingly with a less anhyzer angle than you want it to pivot out with. You can experiment with angle by simulating a disc pivot and with a little practice you should be able to find the angle that works best for you to have the disc release at your desired anhyzer angle.

Pull from higher up and release higher up.Possible Fix:
Typical anhyzer form has the release point higher up than on a regular straight. You will still want to have the disc pull through on a level plane so you will also want to start from higher up during your backswing. Most of the pro players I have spoken with have said the ideal height for a long anhyzer is approximately head high which is probably quite a bit higher than your usual release point.

Roll your wrist over.Possible Fix:
Wrist roll is your tendency to turn your wrist/forearm during or immediately after release. This usually happens when your arm is moving very fast so it might be a tough one to spot. If you go through your motion in slow-motion, open your hand like you are going to shake hands with someone. On a release with no wrist roll your palm should be facing somewhat towards you. This is ideal for a line drive shot. For an anhyzer/turnover, you will want to roll your wrist over. This will end with your palm is facing up and result of this is that discs are more likely to turn over and may help achieve an anhyzer angle. The timing and magnitude of the roll will effect the flight and severe early wrist rolls can lead to grip lock or unintentional rollers.

After reading a Q & A with Swedish pro Tomas Ekstrom, he describes a way of adding snap to an anhyzer throw. Start with the disc in your hand in a position like you have just unlocked a door (aka roll your wrist counter-clockwise). Begin your throw and right near the release point, lock the door. This should roll your wrist over and add a bit of "flick" to the motion.

Adjust the angle of your footwork.Possible Fix:
The line your steps take during your x-step is very important based upon what type of shot you want to throw. Walk through your x-step and see how far you move to either side while you step forwards. Most people will try to stay centered on the teepad as they go through their steps. If you want to make it easier to throw an anhyzer try starting back right and finishing front left while keeping your forward movement perpendicular to the target, this should close your upper body more and allow a higher anhyzer release off to the left.
Keep your elbow closer to your body.Possible Fix:
A strong anhyzer throw requires a slight tilt of the body to the right, usually in the form of an arched back. A common problem that occurs with anhyzer throws is a weak throw that won't hold a turnover line. This is often caused by the arm's tendency to hold the disc straight on its pull line while your upper body tilts and this will cause your arm to get too far away from your body, reducing your power. In order to achieve a strong anhyzer throw, you will need to keep the disc close to your body and concentrate on keeping your elbow in tighter. The elbow in closer will ensure that you can get a strong pull on the disc and get a lot of snap on it with an anhyzer line. Remember though that an anhyer/turnover throw that finishes to the right will not get you as much D as straighter throws or s-curves since you lose the glide you get during the fade period of the disc's flight.

Your grip has improper thumb pressure.Possible Fix:
A solid, strong grip will have pressure between the index finger and the ball of your thumb or lower (between the ball of the thumb and the joint). A common grip problem is applying thumb pressure with the very tip of the thumb. Not only is this grip weaker but it also causes the pressure to be applied at an angle that will effect the disc orientation at the release. The end result is a dipping of the back and outter edges of the disc resulting in a nose up release at a lower (more hyzer) angle than your original disc orientation. If this is the problem you probably have trouble getting a disc to hold a turnover line as it will tend to want to stall and flex early. Changing your thumb pressure to having your thumb more flat and pressure from the ball of your thumb or lower should help to achieve a nose down release on your desired angle.

Your thumb placement is not far enough forward.Possible Fix:
Your thumb pressure and its pinch with the index finger is one of the important factors in controlling nose angle. If you are having trouble getting the nose down on your throws, you may want to try moving your thumb pressure forward. This "forward" refers to the relative placement of the pressure point of the thumb. If your thumb is directly opposed to your index finger's contact point, moving the thumb forward will move the pressure point of the thumb beyond the index finger's point of contact while keeping the thumb the same distance from the edge of the disc. The forward pressure will aid in pushing the nose down during the disc pivot.

What kind of line makes the disc fly straight?Possible Fix:
If your normal disc line consists of s-curves or hyzers and you just can't seem to get the disc to hold a straight flight path, here is a suggestion for what is probably the straighest flight path you can achieve. Rather than trying to throw everything flat which will usually result in a line-drive s-curve, try flattening out a hyzer throw. The disc will rise as it flattens and then "plane out" and hold that line until it slows down. This may involve throwing a more understable disc and making a couple of adjustments. Here are some suggestions to make this process a little easier.

Keep your right shoulder lower than your left.

Make sure you get the nose down.

Find the appropriate disc orientation to get the right amount of hyzer angle on the disc pivot.

Keep it low as the disc will rise as it flattens.

X-step from back right to front left.

Make sure you get a clean release with no flutter.

Be smooth and don't try to overpower it.

Throw the least overstable disc you can control in this manner.

The disc flies straight in the air but rarely online with my target.Possible Fix:
While inaccuracy of your throws are often problems relating to timing and body rotation, there is a simple check you can do to see if it is caused by your grip and disc orientation. During your pull through, the disc should be following "behind" your hand in order to get a consistent release point. If your hand is on the outside of the disc during the pull, a repeatable, consistent release is more difficult and you can find yourself spraying your throws off to the left and right.

A flattened hyzer with a stable disc is going to give the straightest possible flight path for the duration of a disc's flight. I found early on it was difficult to conceptualize the form and line for this type of throw and the many things that can go wrong. I know everyone's level of power and amount of spin on the disc will vary but here are some basic things to look for if you are struggling to flatten a hyzer.

You are exposing the flight plate of the disc on release.Possible Fix:
A straight-flying flattened hyzer will require a different type of release than a standard sweep hyzer. A sweep hyzer starts to hook from the start because some of the flight plate is exposed upon release and the disc will immediately begin to slow. For a disc to fly straight on line and lift and flatten under its own power it requires the disc to leave your hand without any flight plate exposed. The disc will have a certain level of hyzer angle but it must be released "flat" to "nose down" on that angle and in a line directed straight at your target.

Extreme hyzer angles will require a considerable amount of body shift. If you go through the motions of this you will need to bend at the waist and lean over the disc to keep the disc oriented on a plane perpendicular to your target. The greater the hyzer angle, the more you will have to lean your upper body over the disc. An accurate throw will require you to have your weight centered over your pivot foot during the throw so you may have to adjust your footwork to accomplish this. If you get enough spin on the disc, it should flatten and "plane out" and fly straight for most of it's flight with a bit of fade at the end. To throw higher, add more hyzer as the disc will rising until it flattens. If you are gettng enough power to flatten the disc and it's not a disc that is too overstable, you may be rolling your wrist under.

You are rolling your wrist under.Possible Fix:
Wrist roll is your tendency to turn your wrist/forearm during or immediately after release. This usually happens when your arm is moving very fast so it might be a tough one to spot. If you go through your motion in slow-motion, open your hand like you are going to shake hands with someone. On a release with no wrist roll your palm should be facing somewhat towards you. This is ideal for a line drive shot. If your palm is facing down, you are rolling your wrist under. The result of this is an exposure of the flight plate forwards and the resulting throw will be a hyzer. The timing and magnitude of the roll will effect the flight and severe early wrist rolls can lead in stall outs. Wrist roll under is often used on a throw like a knife hyzer, but it won't work for a straight throw. On the other end of things, rolling the wrist too far over is shown with the palm facing up. This will make the disc turn over.

When attempting to flatten a hyzer, a wrist roll under will cause the flight plate to be exposed very early on in the discs flight. While this is good for a sweeping hyzer or knife hyzer, it will require a great deal of power to flatten this type of a throw unless you are throwing a very understable disc. Even in that scenario, the disc will lose most of its speed early on and rise very high in the air sort of like a stall out. In order to give the disc its proper orientation during release you will need to have no wrist roll or a wrist roll over. A release with no roll will give the straightest flight while a wrist roll over will make an understable disc fly straight at the beginning and turn late.

You aren't getting enough power on the throw.Possible Fix:
Flattening a hyzer will require a good amount of power on your throw. The more spin you can get on the disc, the faster it will flatten and the straighter it will fly with the least amount of turn and fade. This type of throw will generally require at least 250' of power to flatten out most of today's newer drivers. I have discussed possible causes of power loss and the possibility of disc stability if you find yourself unable to achieve this type of throw even with the correct disc orientation.

You are throwing too overstable a disc.Possible Fix:
This is an easy fix as it only requires an equipment change. I know that Mr. Top Pro might be able to throw a 450' flattened hyzer with Popular Driver X but if you are reading this you probably aren't quite there yet. Learning to throw a flattened hyzer is most easily accomplished with discs that are very understable and take very little power to flatten. Popular very understable or roller discs are often a good choice for this as they take very little effort to flatten and you can concentrate more on technique than on trying to crush the throw. Although they may be too understable for max distance drives they can help provide a solid foundation. As you get proficient in this technique you will find it handy to throw very understable discs on shots that require a straight flight and a turn later in their flight.

Your grip has improper thumb pressure.Possible Fix:
A solid, strong grip will have pressure between the index finger and the ball of your thumb or lower (between the ball of the thumb and the joint). A common grip problem is applying thumb pressure with the very tip of the thumb. Not only is this grip weaker but it also causes the pressure to be applied at an angle that will effect the disc orientation at the release. The end result is a dipping of the back and outter edges of the disc resulting in exposure of the flight plate with a nose up release at a hyzer angle greater than the initial disc orientation. Attempting to flatten a hyzer with this line will be difficult as the disc will fly with an air bounce. Although it is still possible to flatten the disc out by throwing more understable discs or if you have enough power, however, it will not give the same kind of distance or predictability of a nose down hyzer. Adjusting your thumb pressure to having your thumb more flat and pressure from the ball of your thumb or lower should help correct this shift of the disc orientation.

Your thumb placement is not far enough forward.Possible Fix:
Your thumb pressure and its pinch with the index finger is one of the important factors in controlling nose angle. If you are having trouble getting the nose down on your throws, you may want to try moving your thumb pressure forward. This "forward" refers to the relative placement of the pressure point of the thumb. If your thumb is directly opposed to your index finger's contact point, moving the thumb forward will move the pressure point of the thumb beyond the index finger's point of contact while keeping the thumb the same distance from the edge of the disc. The forward pressure will aid in pushing the nose down during the disc pivot.

Your grip has improper disc placement.Possible Fix:
Your grip should have the edge of the disc going down the seam of your hand in your lower palm's natural central crease. If the disc's contact point is under the seam there will be a tendency for the back edge to dip at the release point. The result of this will be an exposure of the underside of the flight plate and it is likely the disc will fly high and with a hyzer line. This is especially critical since a flattened hyzer must be released flat or nose down, without exposure of the underside of the flight plate. Getting the disc to rest in the seam of your hand may take some grip modification to achieve, especially if you have small hands, but this is one of the important fundamentals that will have an effect on your consistency.